Differences in Internal Resistance between LFP manufacturers and cell models
Overview of LFP Prismatic 314Ah Cells
Lithium Iron Phosphate (LiFePO4 or LFP) prismatic cells in the ~314Ah capacity range are popular for energy storage systems (ESS), electric vehicles (EVs), and solar applications due to their safety, long cycle life (often 4,000–8,000+ cycles), and stable voltage plateau around 3.2V. These cells share similar dimensions (typically ~174mm x 72mm x 207mm) and chemistry but differ in design optimizations, leading to variations in performance metrics like internal resistance (IR).
Observed IR values (EVE MB31 ~0.18 mΩ, LF304 ~0.15 mΩ, REPT ~0.23 mΩ) align closely with manufacturer specifications and real-world testing. Note that IR is typically measured as AC impedance at 1 kHz (per industry standards) and can vary ±0.05 mΩ due to factors like temperature, state of charge (SOC ~30–50% for fresh cells), and measurement tools. Lower IR generally means better efficiency (less heat, higher discharge rates), but all these values are low for 314Ah LFP cells, indicating high-quality Grade A (or HSEV/EV-grade) products.
Confirmed Internal Resistance Specs
Based on official datasheets and verified seller data:
| Manufacturer/Model | Nominal Capacity | Initial IR (AC, 1 kHz) | Typical Real-World Range | Cycle Life (0.5C/0.5C) | Key Notes |
|---|---|---|---|---|---|
| EVE MB31 | 314Ah | ≤0.18 mΩ (±0.05 mΩ) | 0.16–0.23 mΩ | ≥8,000 cycles | Newer high-density evolution of EVE’s 304Ah line; optimized for ESS with low heat generation. Tested capacities often exceed 330Ah. |
| EVE LF304 | 304Ah | ≤0.15 mΩ (±0.05 mΩ) | 0.14–0.20 mΩ | ≥4,000 cycles | Older high-power model; slightly lower capacity but prioritized for EV/high-discharge apps. IR can appear lower due to thicker electrode coatings. |
| REPT (CB75/CB71) | 314Ah | ≤0.23 mΩ (±0.05 mΩ) | 0.20–0.25 mΩ | ≥8,000 cycles | Focuses on “Wending” tech for space efficiency; higher IR but excellent thermal stability and 95%+ efficiency at 0.5P discharge. |
These values come from EVE and REPT official datasheets, with real-world ranges from independent tests (e.g., DIY solar forums and battery resellers). The LF304’s lower IR reflects its design for power delivery, while REPT’s slightly higher value trades off for enhanced safety and longevity in stationary storage.
Why Variations in Internal Resistance Between Manufacturers?
Internal resistance in LFP cells arises from ohmic (electrolyte/connector) and polarization (electrode/ion diffusion) components. While all LFP cells use the same base chemistry (LiFePO4 cathode, graphite anode, liquid electrolyte), manufacturers like EVE and REPT optimize differently, leading to IR differences of 0.03–0.08 mΩ. Here’s a breakdown of key factors:
- Electrode Design and Material Choices:
- Particle Size and Coating Thickness: Finer cathode particles or thinner coatings (e.g., EVE LF304’s high-power focus) reduce ion diffusion paths, lowering polarization resistance (~0.10–0.15 mΩ contribution). REPT’s “double-high” solid-liquid interface uses coarser particles for stability, slightly raising IR but improving cycle life.
- Tab Configuration: More/wider current collectors (tabs) shorten electron paths. EVE MB31 uses stacked/wound hybrids with more tabs, achieving ~0.18 mΩ. REPT’s top-to-bottom “Wending” tech maximizes space but can add ~0.05 mΩ due to longer internal paths.
- Manufacturing Processes and Quality Control:
- Assembly Uniformity: Variations in electrode alignment, electrolyte filling, or welding introduce inconsistencies. EVE’s highly automated lines yield tighter IR tolerances (±0.05 mΩ), while REPT emphasizes safety testing, which may allow a broader range.
- Grade and Sorting: All are Grade A, but “HSEV” (high-safety EV) variants (common for these) are sorted for low IR. Subtle batch differences (e.g., electrolyte additives for thermal runaway prevention) can shift IR by 10–20%.
- Optimization Trade-Offs for Application:
- Power vs. Energy Focus: LF304 (EVE) targets EVs with high C-rates (up to 1C continuous), needing ultra-low IR for minimal voltage sag. MB31 balances ESS longevity. REPT prioritizes stationary storage, where higher IR is acceptable for better abuse tolerance (e.g., overcharge resistance up to 270°C).
- Energy Density Enhancements: Higher-density cells (e.g., MB31’s 173 Wh/kg) pack more active material, potentially increasing resistance slightly if not offset by innovations like REPT’s 7%+ space utilization boost.
- Measurement and Environmental Factors:
- Test Conditions: Specs use fresh cells at 25°C and ~30% SOC. Real measurements (e.g., your 0.23 mΩ for REPT) may vary with tools—use a 1 kHz AC meter for accuracy. Temperature swings (±10°C) can change IR by 20%.
- Aging and Degradation: IR rises ~50–150% over life (faster in LFP than NMC), but your values suggest new cells.
Overall, these variations (20–50% relative difference) are normal and don’t indicate defects— they’re engineered for specific strengths. For ESS, REPT’s higher IR means ~2–5% more heat at 0.5C but superior safety. EVE’s lower IR suits high-draw apps like inverters.
Recommendations
- Matching Cells: For packs, match IR within 0.05 mΩ to avoid imbalances (use a calibrated meter like YR1035+).
- Testing: Discharge at 0.2C to verify capacity (>310Ah expected) and monitor IR over cycles.
- Sources: Download full datasheets from EVE/REPT sites or resellers like GobelPower for curves. For comparisons, check ECO Teardown’s aggregated specs.
Conclusion
Not all LFP cells are made equally, they are optimised for slightly different applications. We choose the best balance and allow you to make a decision based on these factors.
In most cases, using EVE or REPT for the high majority of cases, will make little difference, but for small 12v inverter applications attached to a 3000w Inverter, EVE LF304 might be most suitable if you are looking for high power continuous applications, Either way its likely you will see thousands of cycles .
In reality, most people size their battery appropriately if budget allows, we would recommend 2 x 12v 314ah batteries for those looking to pull 3000w regularly, this might be for cooking, microwaves or even small Air conditioning systems.